1. Field of the Invention
The present invention relates to a catalyst composition and method of making the same, which composition comprises a carrier having a high surface area alumina support coating thereon, on which a catalytic metal is dispersed. More particularly, the present invention relates to alumina catalyst support compositions having initial high catalytic activity and the ability to withstand high temperatures while maintaining good activity in selected reactions over long periods of time.
2. The Prior Art
It is known to the art to prepare catalysts comprising a porous, refractory metal oxide carrier which may be formed as a monolithic skeletal body having a plurality of parallel gas flow passages extending therethrough. The carrier has a high surface area alumina support coating on which one or more platinum group metal catalytic components are dispersed. The carrier may be composed primarily of ceramic-like but porous refractory metal oxides including combined forms, for example, alumina, alumino-silicates, and magnesia-silica-aluminas, e.g. cordierite.
While suitable refractory metal oxides such as cordierite have desirable characteristics such as high temperature and thermal shock resistance which suits them for use as monolithic carriers for catalyst compositions, their surface area is too low to effectively support thereon dispersed catalytic components such as platinum group metals. For this reason, the monolithic carrier is conventionally coated with a thin coating of a high surface area alumina such as gamma alumina, or boehmite on which the catalytic metal component is dispersed. High surface area alumina is prepared by thermally treating any of the various hydrous aluminum oxides or alumina gels at temperatures in excess of 400.degree. C., generally 450.degree.-850.degree. C., with the elimination of at least a portion of the chemically and/or physically combined water and hydrogen groups commonly associated therewith. Generally high surface gamma alumina or boehmite has a surface area in the range of about 100 to about 500 m.sup.2 /g as determined by the BET method.
Platinum metal catalysts supported on high surface alumina have found particular utility as internal combustion engine exhaust gas pollution abatement catalysts. Similar catalyst compositions have been found useful for other applications including, as disclosed in U.S. Pat. No. 3,928,961, catalytically oxidizing a fuel in a combustion operation for purposes of energy production.
In all such applications, a gaseous stream (which may comprise a fuel or combustible pollutants and oxygen in cases where oxidation is to be carried out) is passed over the catalyst, and the reactive components of the gas stream contact the catalytic metal component in order to catalyze the reaction.
A common deficiency associated with alumina supported catalyst systems is the thermal degradation of the catalyst support from extended exposure to the high temperature gases which are encountered in use of the catalyst system. At temperatures of 800.degree. C. or more, the alumina undergoes phase transformation with accompanying rapid loss of surface area and a considerable shrinkage in volume. When the alumina support undergoes this thermally imposed phase transformation, the metal catalysts dispersed on the alumina support either become occluded in the shrunken support with a loss of exposed catalyst surface area or are expelled from the support to encounter drastic sintering, both the occlusion and expulsion phenomena resulting in a significant deactivation in catalyst activity.
The art has attempted a number of methods to ameliorate the thermally induced phase transformation of the alumina support. For example, silicon dioxide, zirconium oxide, alkaline earth oxides and rare earth oxides such as ceria oxide or lanthanum oxide have been admixed with the alumina support material to retard the phase transition of high surface area gamma alumina to low surface area alpha alumina. However, when the so-modified alumina support is exposed to the exhaust gases of a moving vehicle such as in an automobile catalytic muffler in a manifold position where the temperatures may reach levels higher than 1,100.degree. C. for extended periods of time, the modified alumina is often unable to retain its thermal stability properties. In applications such as high temperature steam reformers, where the operating conditions include pressurized steam at temperatures greater than 800.degree. C., the deterioration of the stabilized alumina is accelerated.
Another method devised by the art to improve the thermal stability of alumina supports is represented by British Pat. No. 1,492,274 wherein the alumina support material is impregnated with a low molecular weight silane compound such as alkyl orthosilicates.
According to the patent, the organo-silicon compound is impregnated in the liquid phase on the alumina either in its undiluted liquid form or diluted in an organic solvent at room temperature or by vapor phase deposition at a temperature above the boiling point of the organo-silicon compound. After the impregnation step, excess impregnant is then removed from the alumina at elevated temperatures in an inert atmosphere. Although the use of the organo-silicon compounds disclosed in the British patent are effective to improve the thermal stability of the alumina treated therewith, the disclosed method requires the use of costly, highly flammable organo-silicon compounds and the method to remove the excess compound is wasteful of material and thus relatively costly. Also, an environmental concern is raised with the method disclosed in the British patent as any leaking of the volatile and flammable organo-silicon compounds into the working environment represents a serious safety and environmental hazard.
There is therefore a need in the art to provide a simpler, less costly and less hazardous method whereby the thermal stability of alumina catalyst support materials can be improved.